1. Technical Field of the Invention
This invention relates to an oscillation circuit and electronics using the same and particularly to an oscillation circuit and electronics in which the temperature characteristic of oscillation frequency of a SAW resonator is corrected.
2. Prior Art
In communications equipment such as a portable telephone, the sending and reception of communication data are performed based on the output signals of an oscillator. Therefore, it has been desired to {circle around (1)} stably oscillate in a high frequency band (high frequency stability) and {circle around (2)} stably oscillate in a practical temperature range of the communication equipment (temperature compensated) from a demand of accelerating the communication.
A SAW (Surface Acoustic Wave) resonator is a device for the high-frequency oscillation of an oscillator. The SAW resonator is a resonator making use of such a property that energy concentrates and propagates nearby the surface of an elastomer. More specifically, the SAW resonator functions as a resonator by arranging comb-like electrodes on a piezoelectric substrate and reflecting a surface wave excited by the electrodes to generate a standing wave.
There are various problems in an oscillator which oscillates a high-frequency wave with this SAW resonator.
FIG. 17 is a circuit diagram of a conventional voltage-controlled SAW oscillation circuit (Voltage Controlled SAW Oscillator: VCSO), FIG. 18 is a block diagram showing a construction with an output buffer added to this VCSO. This VCSO is a circuit which variably changes the reactance and change to the phase conditions of an oscillation loop by variably controlling a control voltage Vc applied to a Variable Capacitance Diode Cv and thereby obtains a desirable oscillation frequency F.
However, the VCSO has problems in that not only the oscillation frequency of the VCSO but also the characteristic of control voltage Vc-oscillation frequency F greatly changes by the influence of {circle around (1)} the temperature characteristics of the Variable Capacitance Diode in which capacity greatly fluctuates, especially at a low reverse voltage (large capacity value), {circle around (2)} the temperature-phase characteristic of the active elements and {circle around (3)} the influence of the temperature characteristic of reactance values of the passive parts such as coils, capacitors and so on.
As illustrated in detail, as the temperature characteristic of oscillation frequency F of the VCSO shown in FIG. 19, the oscillation frequency F of the VCSO sometimes greatly changes if the VCSO is at a high-temperature (e.g., 85xc2x0 C.). Moreover, as a temperature characteristic of control voltage Vc-oscillation frequency F of the VCSO shown in FIG. 20, changes of the control voltage Vc-oscillation frequency F characteristic were remarkable in cases of high temperature and low temperature. This caused a problem of circuit control because the control of the control voltage Vc in a high-temperature region is greatly different from the control in other temperature ranges.
Moreover, as shown in FIG. 21, the VCSO was made to have a characteristic where the variable sensitivity of the frequency is small in a region of high oscillation frequency F, i.e., when the control voltage Vc is large and the variable sensitivity of the frequency is large in a region of low oscillation frequency F, i.e., when the control voltage Vc is small by taking vicinity of a serial resonance frequency Fr of the SAW resonator as a boundary. This characteristic is particularly significant when an extension coil is used to expand the frequency variable sensitivity to the quantity of reactance change. Therefore, the variable quantity of the control voltage Vc greatly varies in cases of high frequency and low oscillation frequency F. This also complicates the control problem.
Thus, the conventional VCSO had a problem where the control voltage Vc-oscillation frequency F characteristic greatly changes in some temperature ranges, therefore this VCSO cannot be designed in a loop zone appropriate over the whole temperature range, in designing a PLL for forming a part of PLL circuit synchronous with the frequency of the optical network communication equipment to use as a reference clock source.
Furthermore, as shown in FIG. 22, a conventional oscillation circuit using a SAW resonance is provided with a temperature-compensating circuit 107 composed of resistances 101-104 and heat-sensitive resistors 105, 106 such as thermistors or the like and the temperature-compensating circuit 107 has a temperature compensated SAW oscillation circuit (Temperature Compensated SAW oscillator: TCSO) 100 which changes the control voltage applied to a Variable Capacitance Diode 109 according to an ambient temperature and keeps the oscillation frequency nearly constant. However, this TCSO 100 had problems that the circuit scale is large and miniaturization is difficult because it has the temperature-compensating circuit.
Still more, the construction with the exception of the temperature-compensating circuit 107 of the TCSO 100 shown in FIG. 22 is of a SAW resonator 110, condensers 108, 111, 112, 118, resistances 113, 114, 116, a transistor 115, and a Zener diode 117.
On the other hand, in addition to the method of temperature compensation with the temperature-compensating circuit 107, there is a method wherein the frequency-temperature characteristic of a SAW resonator is corrected by using capacitive elements (condensers) with prescribed capacity-temperature characteristics, as load capacity of the SAW resonator. The method wherein the frequency-temperature characteristic of the SAW resonator is corrected by using capacitive elements with capacity-temperature characteristics is illustrated below.
FIG. 23 is a graph showing the temperature characteristics of the oscillation frequency of a SAW oscillator. As shown in this graph, the temperature characteristics of the oscillation frequency of a SAW oscillator can be nearly expressed by a negative quadratic curve. The curve has a characteristic that the oscillation frequency maximizes at a certain temperature T0 (called a frequency summit temperature T0 hereafter) and the frequency decreases if the temperature changes. On the other hand, there is a relationship of an inverse proportion between the oscillation frequency and the load capacity of the SAW oscillator, if the load capacity reduces, the oscillation frequency rises.
In the case of this SAW oscillator, as shown in FIG. 24, a capacitive element having a capacity-temperature characteristic that the capacity maximizes at the frequency summit temperature T0 is used. As shown in FIG. 25, the temperature characteristics of the oscillation frequency of the SAW oscillator are in a narrow temperature range with the frequency summit temperature T0 at the center, but it is also possible to correct the oscillation frequency to a nearly constant frequency in such a temperature range.
However, this method has a problem, in that the frequency can be kept constant only in a narrow temperature range with the frequency summit temperature T0 at the center, therefore the frequency greatly changes at a high temperature or a low temperature away from the frequency summit temperature T0.
This invention was made to solve the problems in the above-mentioned prior art and is aimed at providing an oscillation circuit with improved temperature characteristics in a wide temperature range, especially in a high temperature range and the electronics using this oscillation circuit.
To solve the above problems, this invention provides an oscillation circuit characterized by a positive feedback oscillation loop constructed by an amplifier, a SAW resonator with a prescribed oscillation frequency, a phase-shifting circuit for outputting the phase of an input signal with a prescribed shift as an output signal and a tank circuit composed of inductance elements and capacitive elements, and an NTC thermistor having a negative temperature characteristic, connected in parallel to the tank circuit.
The above construction improves the frequency-temperature characteristics of a feedback type VCSO and gives an oscillation circuit with a stabilized frequency even if the surroundings are at a high-temperature because the NTC thermistor functions to increase the amount of phase compensation of the tank circuit in a high-temperature range.
In the above construction, the amplifier may also be a differential amplifier having an inverting input terminal and a non-inverting input terminal in which a bias voltage is input into either side of the inverting input terminal or the non-inverting input terminal and the other side functions as the input end of the positive feedback oscillation loop.
The above construction is characterized by the tank circuit being connected between the inverting input terminal and the non-inverting input terminal of the differential amplifier.
In an oscillation circuit having an amplifier, a resonator having quadratic frequency-temperature characteristics, a feedback amplifier circuit for oscillating the resonator and a tank circuit having frequency selectivity at a nearby desirable frequency, this invention provides an oscillation circuit characterized by any one or more of the capacitive elements constructing the tank circuit having a capacity-temperature characteristic for correcting the quadratic frequency-temperature characteristics of the resonator.
According to the above construction, the quantity of the frequency change of a resonator given by the capacity change of the capacitive elements constructing the tank circuit is much more than the quantity of the frequency change of the resonator given by the capacity change of the other capacitive elements. Therefore the quadratic frequency-temperature characteristic of the resonator can be corrected in a wide temperature range by the capacitive elements constructing the tank circuit having the capacity temperature characteristic for correcting the quadratic frequency-temperature of the resonator.
The above construction is characterized in that any one or more of capacitive elements constructing the tank circuit have the capacity-temperature characteristic with the maximum capacity in the vicinity of a temperature at which the maximum oscillation frequency of the resonator is obtained. Therefore, the case where the temperature is overcompensated or under-compensated in the range of high temperature or low temperature is prevented from occurring, and the maximum effect in temperature compensation is obtained.
In the above construction, the oscillation circuit may also have a phase-shifting circuit for changing the phase of a reference signal flowing into the positive feedback oscillation loop by a prescribed quantity and outputting it to satisfy phase the conditions of the oscillation circuit. Therefore, the phase condition of the oscillation circuit is easily satisfied.
In the above construction, the phase-shifting circuit may also have a construction where the quantity of phase shift can be adjusted with an external signal. Therefore, the oscillation frequency of above oscillation circuit is adjusted with an external signal (e.g., control voltage Vc) arbitrarily.
In the above construction, it is preferable that the amplifier is a differential amplifier using an ECL line receiver from viewpoints of high-speed actuations and consumed power reduction.
This invention can provide an optical interface module which provides electronics for use in a high-temperature range to perform data sending and reception stably, e.g., via an optical network without being affected by ambient temperature by use of these oscillation circuits.